The present invention relates to photovoltaic devices and more particularly to photovoltaic devices having semiconductor bodies with increased absorption coefficients.
Photovoltaic devices such as solar cells are capable of converting solar radiation energy into usable electrical energy. This conversion of energy occurs as a result of what is well known in the solar cell field as the "photovoltaic effect." Two processes are involved in the "photovoltaic" effect. The first process is the generation in the semiconductor material of electrons and holes as a result of the materials absorption of solar radiation. Secondly, the electrons and holes are separated at some built-in electric field in the solar cell device, and it is this separation which results in the generation of an electrical current. A built-in electric field can be generated in a solar cell by, for example, a P-N junction.
For the electrons and holes to be useful in current generation it is desirable that they be generated within a diffusion length of the P-N junction. As is well known in the art, the probability of a generated hole or electron crossing a P-N junction is reduced if it is generated beyond a diffusion length. Thus, a semiconductor material, such as silicon, with an improved solar radiation absorption coefficient, would increase the efficiency of a silicon solar cell device, by absorbing more radiation within a diffusion length of the junction.
Also a problem encountered in the field of solar cells is the cost of producing electrical energy on a competitive basis with respect to other means of electrical energy generation. One of the largest expenses involved in solar cell manufacture is the cost of the semiconductor material of the solar cell body, which is typically single crystalline silicon. Silicon has good photovoltaic properties since it can absorb radiation over most of the solar radiation spectrum and has a long carrier lifetime. However, the body of a silicon solar cell is very thick, i.e. about 50 micrometers or more, to ensure sufficient absorption of solar radiation. Naturally, the more semiconductor material needed the higher the cost of a solar cell. Therefore, another desirable effect of improving the absorption coefficient of a semiconductor material, such as silicon, would be to reduce the thickness of the silicon bodies in photovoltaic devices.